In this ethnographic study of summer undergraduate research (UR) experiences at four liberal arts colleges, where faculty and students work collaboratively on a project of mutual interest in an apprenticeship of authentic science research work, analysis of the accounts of faculty and student participants yields comparative insights into the structural elements of this form of UR program and its benefits for students. Comparison of the perspectives of faculty and their students revealed considerable agreement on the nature, range, and extent of students' UR gains. Specific student gains relating to the process of "becoming a scientist" were described and illustrated by both groups. Faculty framed these gains as part of professional socialization into the sciences. In contrast, students emphasized their personal and intellectual development, with little awareness of their socialization into professional practice. Viewing study findings through the lens of social constructivist learning theories demonstrates that the characteristics of these UR programs, how faculty practice UR in these colleges, and students' outcomes--including cognitive and personal growth and the development of a professional identity--strongly exemplify many facets of these theories, particularly, student-centered and situated learning as part of cognitive apprenticeship in a community of practice.